Electrode configuration for electrophotography

ABSTRACT

A PERSISTENT INTERNAL POLARIZATION (PIP) ELECTROPHOTOGRAPHY PRINTING OR COPYING SYSTEM WHEREIN A PAIR OF OPPOSITE POLARITY ELECTRODES, EACH INCLUDING A PLURALITY OF ELEMENTS DISPOSED IN SPACED ARRAY ALTERNATING WITH EACH OTHER IN THE SAME PLANE, ARE UTILIZED IN A COMBINATION WITH A PIP LAYER SUCH THAT AN ELECTRIC FIELD MAY BE SIMULTANEOUSLY APPLIED TO THE PIP LAYER WHILE PERMITTING LIGHT RADIATION TO REACH THE PIP LAYER. IN THE SPECIFIC PRACTICE, THE PAIR OF ELECTRODES ARE POSITIONED ON TOP OF OR EMBEDDED IN THE SURFACE OF THE PIP LAYER WHICH IS TO BE TONED; THAT IS THE TOP SURFACE.   D R A W I N G

Oct. 24, 1972 J. D. GRI ER ELECTRODE CONFIGURATION FOR ELECTROPHOTOGRAPHY Filed Jan. 14, 1971 FIG.

. FIG. 2

U.S. Cl. 96-1 R 5 Claims ABSTRACT OF THE DISCLOSURE A persistent internal polarization (PIP) electrophotography printing or copying system wherein a pair of opposite polarity electrodes, each including a plurality of elements disposed in spaced array alternating with each other in the same plane, are utilized in combination with a PIP layer such that an electric field may be simultaneously applied to the PIP layer while permitting light radiation to reach the PIP layer. In the specific practice, the pair of electrodes are positioned on top of or embedded in the surface of the PIP layer which is to be toned; that is, the top surface.

CROSS REFERENCE TO RELATED APPLICATIONS This is a division of patent application Ser. No. 803,353, filed Feb. 28, 1969, now US. Pat. No. 3,597,073 and titled Electrode Configuration for Electrophotography."

BACKGROUND OF THE INVENTION This invention relates to novel apparatus and process for practicing electrophotographic printing or copying. More particularly, this invention relates to printing apparatus and process utilizing photoconductive insulating materials and the principles of persistent internal polarization.

Persistent internal polarization (abbreviated herein as PIP) involves the separation of positive and negative charges in a photoconductive insulating material by subjecting it to irradiation and an electric field. The charges are subsequently trapped and remain fixed or frozen so as to form an internal polarization field for a period of time suflicient to permit toning. PIP and the theory thereof are well known in the electrophotography art. See, for example, Electrophotography, by R. M. Schatfert, The Focal Press, London and New York (1965), pp. 5977, and Persistent Internal Polarization, by Kallman and Rosenberg, The Physical Review, vol. 97, No. 5 (Mar. 15, 1955), pp. 1596-1610, both of which are incorporated herein by reference.

In general, a PIP electrophotography system includes a layer of photoconductive insulating material sandwiched between a pair of field producing electrodes. The phenomenon of PIP can be achieved in any material which exhibits the following characteristics:

(1) The material must have a high resistivity in the dark (a low density of free charge carriers), whereby it is a good insulator in the absence of irradiation.

(2) The material must be photoconductive. In other words, it must have decreased resistivity when excited with appropriate radiation.

Thus, a PIP material is one which will become persistently internally polarized due to the separation of positive and negative charges when it is subjected to irradiation and the action of an electric field.

Typical PIP materials contemplated herein comprise binder dispersions of photoconductors and binder free films of photoconductors.

United States Patent O E CC Examples of inorganic photoconductors contemplated in the process of this invention include, not by way of limitation, appropriately activated zinc sulfide, cadium sulfide, zinc selenide, cadmium selenide, cadmium oxide, zinc-cadmium selenides, and zinc-cadmium sulfides. Examples of organic photoconductors include anthracene, chrysene, and poly (vinylcarbazole) Examples of resin binders contemplated herein include, not by way of limitation, cellulose acetate, cellulose ether, cellulose ester, silicones, vinyl resins, alkyds, and/ or epoxy resins. When using sulfide photoconductors, it is suggested that glass binders be used if they are low melting compositions which contain no lead to poison the sulfide phosphors.

The irradiation of the PIP material can be accomplished by means of any form of electromagnetic or particulate radiation or energy, visible or invisible, which will excite the PIP material so as to permit charge movement in an electric field. Such radiation includes, not by way of limitation, visible light, infrared, ultraviolet, X-rays, gamma rays, and beta rays. For printing or copying purposes, the typical radiation is light in the visible range.

In the prior electrophotographic printing and copying art, simultaneous application of the electric field and the light from an image to a PIP material has been obtained by means of a pair of conductive electrodes separated by a layer of PIP material. Such an arrangement can be difiicult to handle as the substrate layer must be conductive and variations in the thickness and composition of the PIP layer can afiect the performance of the system.

SUMMARY OF THE INVENTION In accordance with this invention, there is provided a new and novel electrode configuration which overcomes the aforementioned disadvantages associated with the prior art devices. More particularly, this invention provides a discontinuous electrode configuration wherein a pair of opposite polarity electrodes are positioned in the same plane or the same side of the PIP layer.

In accordance with a specific embodiment of this invention, each of the opposite polarity electrodes includes a plurality of elements disposed in spaced array alternating with each other in the same plane and are attached to or embedded in the top surface of the PIP layer.

When it is desired to form a latent electrostatic image in the PIP material, an electric field is applied and the PIP materials are exposed to an image or other pattern of activating radiation. The electrode elements in the illuminated portions will assume PIP charge of sign opposite to that applied; thus, one half will be positively and one half negatively charged.

If the exposure to the image is continued for a suflicient time period, the irradiated area of the PIP layer will polarize. Thus, the image is simulated by an internal latent electrostatic image or pattern detectable at the surface of the PIP material.

This latent electrostatic image is subsequently developed with charged or dipolar toner particles so as to produce a visible reproduction of the image which is capable of being viewed, photographed, or transferred, utilizing known methods in the electrophotography printing or copying art.

It should be noted that, due to the characteristics of the PIP material, the latent electrostatic image produced in the PIP material will typically remain fixed such that a finite number of reproductions can be made. The image can be erased by overall irradiation, thereby returning the PIP material to a neutral condition capable of being used for the formation of a new electrostatic image.

In operation, an electric field is applied between the pair of opposite polarity discontinuous electrodes and the radiation is impinged upon the PIP layer through the discontinuous electrodes. Thus, an electrostatic latent image is formed which is capable of being toned and the toner image transferred.

Other features and advtntages of the subject invention will become obvious to those skilled in the art upon reference to the following detailed description and the drawings illustrating a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a schematic view of a PIP system having a pair of discontinuous electrodes positioned on the top surface of a PIP layer and being flooded with image-wise radiation in accordance with this invention.

FIG. 2 is an end view of the PIP system of FIG. 1 showing schematically the electrode configuration.

DESCRIPTION OF A PREFERRED EMBODIMENTS In the drawings, the numeral refers to a body of PIP material as previously described. The PIP body 10 has attached to it or embedded in it a pair of electrodes 12 and 14 whcih are connected to a DC source E. For the purposes of explanation, the electrode 12 (as viewed in FIG. 2) is connected to the positive terminal of the DC source E and, accordingly, the electrode 14 is connected to the negative terminal of the DC source E.

The pair of electrodes 12 and 14 are discontinuous in form and each electrode may take the form of a comb structure which is attached to or embedded in the top surface of the PIP body 10. It is suggested that the comb structure of each electrode include a plurality of elements 16 disposed in spaced array and alternating with each other in the same plane. Thus, the desired configuration has a plurality of alternating electrode elements 16 of opposite polarity adjacent to each other in the same plane. Therefore, both electrodes 12 and 14 are positioned on the same side of the PIP body 10, thereby eliminating the need for a back electrode or conductive substrate on the other side of the PIP body 10.

When the system is subjected to image-wise radiation (circle 18) and an electric field, the PIP body 10 reacts as shown in FIG. 2. Only in the areas subjected to radiation from the image (i.e., those within the circle of image-wise radiation 18) are mobile charges produced which result in internal polarization under the force of the field. The PIP system has thus produced a latent electrostatic image (as represented schematically by the charges within the circle of imagewise radiation 18 in FIG. 2) which is capable of being transferred and printed through the use of charged electroscopic powder. To erase the electrostatic image, the PIP body may be flooded with an overall radiation.

A continuous electrode has an inherent disadvantage in that it must be removed in order to develop the latent electrostatic image as it acts as an insulator between the latent electrostatic image and the electroscopic particles because of the presence of bound image charges. In contrast to this, the discontinuous comb-shaped electrodes 12 and 14 of this invention with their high percentage of openings do not insulate the electroscopic particles from the fringing fields of the latent electrostatic image and, therefore, need not be removed during the transfer or printing stages. Thus, a discontinuous electrode has a distinct advantage in that its nonremovability saves considerable time and facilitates the transfer necessary to use a PIP system in a printing or copying machine.

In commonly available copying machines, it has been found that the toning of a large solid area often results in decreased toner density; that is, deterioration of the image in areas furthest away from the edges. In other words, the middle portions of a large solid area which has been toned and transferred from such an image often appear less distinct than do the edge portions. The use of a discontinuous electrode (such as the combs 12 and 14) electrically breaks up the large areas, thereby resulting in uniform development over the large areas of the latent image.

An advantage of nonremovable electrodes, such as the attached or embedded comb-shaped electrodes 12 and 14, is their ability to avoid dust and toner collection between the electrode and the PIP layer. Removable electrodes frequently pick up dust particles and other foreign matter which, when positioned between the electrode and the PIP layer, distort the field lines. Nonremovable electrodes embedded in the top surface of the PIP layer such as the comb-shaped electrodes 12 and 14 of this invention completely eliminate the possibility of dust particles gathering between the electrodes and the PIP layer, thereby insuring against distortion of the field lines.

Given the pair of nonremovable, comb-shaped electrodes utilized in combination with a layer of PIP material, there are a number of variations which can be utilized within the scope of this invention. Either a charged toner or a dipolar toner can be used. It should be understood that dipolar toner would tone both polarities, thereby maintaining image resolution at the same value as line resolution. The same result can be accomplished by the use of two charged toners, one of each sign, either in mixture or sequentially.

The comb-shaped electrodes 12 and 14 may be formed of thin conductive metal strips attached to or embedded in the PIP layer surface; or they may be evaporated metallic layers put down onto the PIP layer surface by standard evaporization techniques. Also, they may be printed with PAF materials and techniques.

Thus, it can be seen that this invention provides a pair of conductive, nonremovable, comb-shaped electrodes which are positioned in the same plane on the same side of the PIP layer and each includes a plurality of elements disposed in spaced array alternating with each other. Such pair of discontinuous comb-shaped electrodes can be used to simultaneously apply an electric field and permit radiation to reach the PIP layer and do not have to be removed for image toning and transfer. Since both electrodes are positioned on the same side of the PIP layer, this invention provides a compact unit which eliminates the need for a conductive substrate or back electrode and which eliminates the difficulties associated with variable thicknesses and surface irregularities in PIP materials.

It should be noted that although this invention has been described in connection with a planar system, it is well suited to be used in conjunction with a rotary drum system and may be connected so that the drum is commutatable. It would be contemplated that the comb electrodes would be appropriately segmented so that various portions of the drum could be performing different functions at the same time.

Although this invention has been described and illustrated in detail by reference to a specific embodiment, it will be obvious to those skilled in the art that many changes and modifications may be made thereto without departing from the scope of this invention. Therefore, this invention is not intended to be limited except as defined in the claims hereinafter.

I claim:

1. In a PIP electrophotographic printing or copying process wherein a photoconductive layer exhibiting PIP is subjected to an electric field and irradiated, the improvement which comprises irradiating the PIP layer through the open interstices of a pair of discontinuous electrodes, said pair of discontinuous electrodes being positioned in the same plane on the same surface of the photoconductive layer so that a potential difference between said pair of discontinuous electrodes exists in a plane parallel to said same surface of said photoconductive layer.

2. A PIP electrophotographic printing or copying process as set forth in claim 1 wherein said pair of discontinuous electrodes are comb-shaped and each of said pair 5 of discontinuous electrodes includes a plurality of elements disposed in spaced array alternating with each other in the same plane.

3. A process as set forth in claim 2 wherein each of said pair of discontinuous electrodes is attached to the same surface of said photoconductive body.

4. A process as set forth in claim 3 wherein each of said pair of discontinuous electrodes are at least partially embedded in the same surface of said photoconductive body.

5." A process as set forth in claim 4 wherein each of said pair of discontinuous electrodes are completely embedded in the same surface of said photoconductive body.

References Cited 5 UNITED STATES PATENTS 2,912,592 11/ 1959 Mayer 250-211 3,005,707 10/1961 Kallmann et al 96-1 3,137,762 6/1964 Baumgartner et a1. 88-6l GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner 

